Japan's Carabus male beetles mate with females by inserting their chitin-covered appendage. Two pieces of the appendage break off: a sperm delivery tube and a copulatory piece. Researchers identified several genes that control the size of the male beetle's "piece" and the female beetle's pocket. Initial hypotheses demonstrated that the male and female's genes for genitals should be connected through coevolution which should show in their genome. In other words, the size and shapes of genitalia in species should be analogous because the same genes influence male and female dimorphism. However, researchers found that there is much genetic diversity between the genes that determine these beetle's genitalia sizes. The discovery is a significant case in speciation because it contributes to an old speciation model called the "Lock-and-Key Hypothesis".
The Lock-and-Key Hypothesis is a method of reproductive isolation, specifically mechanical isolation which separates a species from mating. For example, the beetles described in the article have such a large genetic diversity contributing to genitalia that some of the male appendages do not fit into females. The article states, "Out-of-sync sizes can cause ruptures, snap-offs and generally low numbers of offspring." Talk about painful! These beetles are now contributing to hybridization which can eventually evolve into a whole other species altogether. Genetic hybrids tend to be infertile, but researchers found that the hybrids of two different beetle species in Japan can produce offspring.
It is truly interesting to see evolution in motion especially with the Lock-and-Key Hypothesis. It's no secret that genitalia in male species may not fit into a female in either the same species or another. The Lock-and-Key Hypothesis was proposed over a hundred years ago, but it's often overlooked because other reproductive isolations such as gametic isolation or chromosomal differences are better modes of preventing hybridization. The discovery here is fascinating because genetic diversity actually prevented some of these beetles from mating within their own species.